A revolution in the information display technology began in the early 1970s with the invention of the liquid crystal display (LCD). Because the LCD is a flat-panel display of light weight and low power which provides a visual read out that conforms to the small size, weight and battery demands of a handheld electronic device, this display technology enabled a new broad class of handheld and other portable products. Commercially, the LCD first appeared in volume as a digital readout on wrist watches, then on instruments and later, enabled the laptop computer, personal data assistant and many other digital devices. Today LCD technology is even replacing cathode ray tubes in televisions and PCs.
LCDs that appear on TVs, PCs, handheld devices, etc. are electronically addressed with an image by a multitude of direct electrical connections (interconnects) between the integrated driving circuits (drive chips) and the display electrodes that make up the pixel elements. The electrodes may be the rows and columns of a passive matrix or the data and control lines of an active matrix. The electrodes are connected by interconnects to drive chips which are further connected to and operated by controller circuitry. The drive chips and control circuitry make up a substantial cost of the display and interconnects to the drive chips must be manufactured with high yield so that there are no unaddressed pixels in the display image. It is largely because of these problems that LCDs have not been popular for such applications as point of sales signs in supermarkets, body worn displays such as badges and other applications where either the complexity of wiring up a collection of displays or the cost, weight and bulkiness of the electronics prevents their use.
Reflective bistable cholesteric displays were invented in 1991 whereby an image could be written on the display and the image retained indefinitely without any applied power [see: J. W. Doane and A. Khan “Cholesteric Liquid Crystals for Flexible Displays” in Flexible Flat-Panel Displays, Edited by G. Crawford, Chapter 17 (John Wiley & Sons, 2005)]. In this case, the drive electronics could be removed entirely from the display and a bright, high contrast image could be viewed at wide angles; this would be very effective for such applications as point of sales or body worn displays. However, the drive electronics would need to be reconnected to change the image thus preventing cholesteric displays from being an attractive solution for these applications.
In 1997, H. Yoshida et al. at the Liquid Crystal Institute at Kent State University offered a clever solution to this problem [see: Yoshida et al., “Reflective Display with a Photoconductive Layer and Bistable Reflective Cholesteric Mixture” Journal of the SID, 5/2, 269-274 (1997)]. Borrowing from earlier work on photo activated systems, they incorporated a photoconductive layer between one of the electrodes and the cholesteric liquid crystal layer to create a photoactivated reflective bistable cholesteric display [see: A. M. Lackner, et al., “Photostable Tilted-Perpendicular Alignment of Liquid Crystals for Light Values” Proceedings of the SID, 31/4, 321-325 (1990)]. This concept enabled a display that could be addressed with a high resolution image without any drive chips or control circuitry and with only two electrical interconnects to apply a voltage to simple unpatterned electrodes. This not only eliminated bulky and costly electronics from the display but also avoided the necessity of making electrical connections to a multitude of electrodes. Recently, photoactivated bistable cholesteric displays have been further developed by Fujitsu workers [see: WO 04/029,708 A1] for contrast improvement. Workers at Fuji Xerox Ltd. also extended the photo activation concepts to flexible displays by employing organic photoconductors on the display substrate [see: N. Hiji et al., SID Digest of Papers, Vol. XXXIV, 1560-1563, (2005)]. Fuji Xerox has further developed products from these devices.
Other types of photoactivated cholesteric or chiral nematic display devices have been devised that employ photo sensitive chemical additives that adjust the reflective wavelength of the chiral nematic material in the display. In such a device the photosensitive additive shifts the reflective wavelength of exposed region of the planar texture to create an image avoiding the use of a photoconductive layer.
The idea of a chiral photochemical reaction to change the twist and hence the pitch length of a chiral nematic material goes back as far as 1971 and the studies of Sackman [E. Sackman, J. Chem. Phys. Soc., 93, 7088 (1971)]. Since that time there have been some remarkable advancements in the development of novel chiral materials, [see T. Ikeda, J. Mater. Chem., 13, 2037-2057 (2003)]. Of particular interest are photochemical switches that act both as a chiral agent to induce a cholesteric phase in a nematic liquid crystal and a photo-responsive dopant that can have a pronounced effect on the twist of the cholesteric helix. Many of the studies have involved the photo-responsive azobenzenes with chiral pendants attached to various positions; however the values of the helical twisting powers are low. Recently Pieraccini et al., [S. Pieraccini et al., Chem. Comm., 598-599 (2003); S. Pieraccini et al., Chem Eur. J., 10, 5632-5639 (2004)] synthesized several bis(azo) compounds containing axially chiral binaphthyls which were found to exhibit large twisting powers. One isomer was measured to yield a twisting power of 144 μm−1 and upon irradiating the material alternately at ultraviolet and visible wavelength the twisting power could be switched repeatedly between 75 μm−1 and 105 μm−1. As will be described below, this is an encouraging result for the application proposed here and, advantageously, could be used to switch the reflective wavelength by 30%. Other photoswitching compounds have been examined by Feringa et al. [B. L. Feringa et al., J. Chem. Soc. Chem. Comm., 288, (1993)] who studied the sterically overcrowded alkenes as chiroptical trigger molecules. Upon irradiation at the appropriate wavelength, these molecules undergo cis-trans photoisomerizations that simultaneously result in helix reversal. Other photochemicals include the diarylethenes known for high thermal stability of both isomers and high fatigue resistance (high durability) and others that can be found in excellent reviews on the subject [see B. L. Feringa et al., Chemical Reviews, 100, 1789-1816 (2000)].
More recently, T. E. Welter et al. (U.S. Patent Application Publication 2006/0124899) have disclosed a series on photochemically active chiral compounds for use in shifting the Bragg reflective peak in the planar texture of a chiral nematic material.
Photochemical materials have been incorporated into polymers and used for making irreversible images on polymer films [see M. Brehmer et al., Advanced Materials, 10, 1438-1441 (1998) and P. van de Witte et al. Journal of Applied Physics, 85, 7517-7521 (1999). In this application, films are formed from the materials which can then be irradiated through a mask to create an image replicating the mask on the film. The use of polymeric materials reduces molecular diffusion allowing the image to be retained on the film for an extended period of time. U.S. Pat. No. 6,723,479 B2 describes means for transferring optically modified films to surfaces of various items.
In the invention reported here the photochemical materials are prepared and used to create reversible images. The images can remain on the display indefinitely without degradation and can be erased and rewritten. New reversible photo responsive chiral materials are disclosed with high twisting power and high sensitivity to ultraviolet of addressing an image on the display. The invention discloses a photosensitive device that can display a high resolution image without the need of attached drive and control electronics substantially reducing the cost of the display unit for use in applications where paper is currently used.